Argon 40 in cogenetic feldspar-mica mineral assemblages

Abstract

K and Ar analyses on cogenetic feldspars and micas from four plutonic bodies, two pegmatites, and one metamorphic terrain indicate that (1) albite and plagioclase have equal or greater argon retentivity than their cogenetic micas, (2) some potash feldspars of Tertiary age may quantitatively retain argon, and (3) intense metamorphic processes can thoroughly degas both feldspars and micas. The K‐Ar ages of the micas and known chronostratigraphic relations indicate that pegmatitic albite, Tertiary plutonic plagioclase, and possibly Tertiary plutonic potash feldspar contain Ar40 in excess of that generated by the decay of their contained K. This excess component is believed to have been occluded within these minerals at the time of their crystallization from the liquid magma. The excess Ar40 contents derived from these measurements is 0.15 to 2.71 × 10−10 mole/gram in albites and plagioclases of Paleozoic and Tertiary age. The amount of excess Ar40 most likely reflects the partial pressure of Ar40 in the lithospheric gases. K‐Ar ages of plagioclase from two Precambrian quartz monzonites are very nearly the same as the K‐Ar ages of cogenetic biotite and muscovite. These results are concordant with Rb‐Sr whole‐rock isochron and mica ages of 1420 m.y. and demonstrate that plagioclase from Precambrian acidic plutonic rocks can be as precise a chronometer as mica minerals.

Geology of a zone of metamorphic core complexes in southeastern Arizona

“…Xenoliths and septa of Precambrian igneous rock throughout the complexes yield middle Tertiary K-Ar ages that are essentially identical to those of the intrusive phases, which indicates that the thermal event(s) responsible for the essentially concordant ages was strong and uniform over large areas, and either occurred in, lasted until, or reoccurred in the middle Tertiary. Some (but not all) of the few Rb-Sr ages for the complexes (Damon and Giletti, 1961;Livingston and others, 1967;Shakel and others, 1972;Shakel, 1972Shakel, , 1974 and U-Th-Pb ages for the cataclastically deformed intrusive phase of the Santa Catalina-Rincon Mountains complex (Catanzaro and Kulp, 1964;Shakel and others, 1977) seem to favor the last two of the preceding three alternatives. However, the Rb-Sr ages show "poor systematics reflecting original differences or lack of isotopic equilibrium" (Shakel and others, 1977), and the U-Th-Pb ages, while reaffirming that a strong and widespread thermal event occurred in or lasted until the middle Tertiary, are not necessarily definitive of the age of the cataclasis or the age of emplacement of the major intrusive bodies involved.…”

Potassium‐argon dates on some Cainozoic volcanic rocks from Northeastern New South Wales

“…Either the alkali olivine basalt has lost some radiogenic argon or the plagioclase phenocrysts of the alkali dolerite possess some excess radiogenic argon (cf. Livingston et al, 1967). The total-rock sample GA 1961 is essentially holocrystalline and almost free of alteration so that its date is likely to be reliable.…”

K—Ar age, composition, and origin of Mesozoic mafic rocks related to Ferrar Group, Pensacola Mountains, Antarctica

“…Using 4OArj3 9Ar age-spectra methods, they show that ages significantly older than 190 m.y., including plagioclase, are anomalous owing to extraneous 40Ar and that bodies in both terranes are approximately contemporaneous in the Early Jurassic. Excess 40Ar is also reported for plagioclase in plutonic bodies (Livingston et al 1967) and probably occurs in plagioclase of the Mt Crillon-La Perouse layered gabbro intrusion of southeast Alaska. Although preliminary paleomagnetic data for this intrusion are compatible with a Precambrian age, and though K-Ar ages of minerals, including plagioclase and pyroxene, show a wide range of 36-250 m.y.…”